This invention relates to a process for preparing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid using a homogeneous catalyst system. More particularly, this invention relates to a process for producing N-phosphonomethylglycine by the oxidation N-phosphonomethyliminodiacetic acid using a salt of a selected metal in the presence of a quinone or quinone derivative.
N-Phosphonomethylglycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. N-Phosphonomethylglycine and its salts are conveniently applied in an aqueous formulation as a postemergent phytotoxicant for the control of numerous plant species. N-phosphonomethylglycine and its salts are characterized by broad spectrum activity, i.e., the controlled growth of a wide variety of plants.
U.S. Pat. No. 3,950,402 to Franz discloses a process for the production of N-phosphonomethylglycine by forming an admixture of N-phosphonomethyliminodiacetic acid, water, and a metallic catalyst selected from the noble metals, heating the admixture to an elevated temperature (greater than 70 C to avoid low yields) and contacting the admixture with a free oxygen-containing gas.
U.S. Pat. No. 3,954,848 to Franz discloses a process for the production of N-phosphonomethylglycine by reacting N-phosphonomethyliminodiacetic acid with an oxidizing agent, such as hydrogen peroxide, in an aqueous acidic medium in the presence of a strong acid at a temperature of from about 70.degree. C. to about 100.degree. C. It is disclosed that one should employ at least 2 moles of the hydrogen peroxide for each mole of the N-phosphonomethyliminodiacetic acid, and preferably more.
Hungarian Patent Application No. 187,347 discloses a process for the preparation of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid with peroxides using a catalytic amount of a metal compound selected from compounds of silver, iron, tin, lead, manganese or molybdenum. Molybdates are preferred. At temperatures lower than 80.degree. C., usually a contaminated end product is obtained. Typically, the reaction is carried out at a temperature of above 80.degree. C. and preferably above 100.degree. C. at pressures exceeding atmospheric, wherein the intermediate N-oxide is decomposed as rapidly as it forms. It is further disclosed that at least two mole equivalents of peroxide should be used for each mole equivalent of N-phosphonomethyliminodiacetic acid.
Although satisfactory results are obtained by the processes of the prior art to prepare N-phosphonomethylglycine using heterogeneous catalysts such as activated carbon or a noble metal on a support, there is now provided a process for preparing N-phosphonomethylglycine using a homogeneous catalyst system which produces outstanding results through high conversions and selectivities, which minimizes the formation of undesirable by-products such as phosphates, and simplifies the separation of the product from the catalyst. The process of the present invention also achieves these results at lower pressures than the molecular oxygen gas processes of the prior art.